[Chapter 7] 7.6 Gateway Routing Daemon

[Chapter 7] 7.6 Gateway Routing Daemon



determining the best route, and combines that information to update the routing table. Before gated

was written, if a UNIX system ran multiple routing protocols each would write routes into the routing

table without knowledge of the other's action. The route found in the table was the last one written not necessarily the best route.

With multiple routing protocols and multiple network interfaces, it is possible for a system to receive

routes to the same destination from different protocols. gated compares these routes and attempts to

select the best one. However, the metrics used by different protocols are not directly comparable.

Each routing protocol has its own metric. It might be a hop count, the delay on the route, or an

arbitrary value set by the administrator. gated needs more than that protocol's metric to select the best

route. It uses its own value to prefer routes from one protocol or interface over another. This value is

called preference.

Preference values help gated combine routing information from several different sources into a single

routing table. Table 7.1 lists the sources from which gated receives routes, and the default preference

given to each source. Preference values range from 0 to 255, with the lowest number indicating the

most preferred route. From this table you can see that gated prefers a route learned from OSPF over

the same route learned from BGP.

Table 7.1: Default Preference Values

Route Type

Default Preference

direct route

0

OSPF

10

Internally generated default 20

ICMP redirect

30

static route

60

Hello protocol

90

RIP

100

OSPF ASE routes

150

BGP

170

EGP

200

Preference can be set in several different configuration statements. It can be used to prefer routes from

one network interface over another, from one protocol over another, or from one remote gateway over

another. Preference values are not transmitted or modified by the protocols. Preference is used only in

the configuration file. In the next section we'll look at the gated configuration file (/etc/gated.conf)

and the configuration commands it contains.



Previous: 7.5 Exterior

Routing Protocols

7.5 Exterior Routing Protocols



TCP/IP Network

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Book Index



Next: 7.7 Configuring gated



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7.7 Configuring gated



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[Chapter 7] 7.5 Exterior Routing Protocols



Previous: 7.4 Interior

Routing Protocols



Chapter 7

Configuring Routing



Next: 7.6 Gateway Routing

Daemon



7.5 Exterior Routing Protocols

Exterior routing protocols are used to exchange routing information between autonomous systems.

The routing information passed between autonomous systems is called reachability information.

Reachability information is simply information about which networks can be reached through a

specific autonomous system.

RFC 1771 defines Border Gateway Protocol, the leading exterior routing protocol, and provides the

following description of the routing function of an autonomous system:

The classic definition of an Autonomous System is a set of routers under a single

technical administration, using an interior gateway protocol and common metrics to

route packets within the AS, and using an exterior gateway protocol to route packets to

other ASs.... The administration of an AS appears to other ASs to have a single coherent

interior routing plan and presents a consistent picture of what networks are reachable

through it. From the standpoint of exterior routing, an AS can be viewed as

monolithic...

Moving routing information into and out of these monoliths is the function of exterior routing

protocols. Exterior routing protocols are also called exterior gateway protocols. Don't confuse an

exterior gateway protocol with the Exterior Gateway Protocol (EGP). EGP is not a generic term; it is a

particular exterior routing protocol, and an old one at that.



7.5.1 Exterior Gateway Protocol

A gateway running EGP announces that it can reach networks that are part of its autonomous system.

It does not announce that it can reach networks outside its autonomous system. For example, the

exterior gateway for our imaginary autonomous system nuts-as can reach the entire Internet through

its external connection, but only one network is contained in its autonomous system. Therefore, it

would only announce one network (172.16.0.0) if it ran EGP.

Before sending routing information, the systems first exchange EGP Hello and I-Heard-You (I-H-U)

messages. These messages establish a dialog between two EGP gateways. Computers communicating

via EGP are called EGP neighbors, and the exchange of Hello and I-H-U messages is called acquiring



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[Chapter 7] 7.5 Exterior Routing Protocols



a neighbor.

Once a neighbor is acquired, routing information is requested via a poll. The neighbor responds by

sending a packet of reachability information called an update. The local system includes the routes

from the update into its local routing table. If the neighbor fails to respond to three consecutive polls,

the system assumes that the neighbor is down and removes the neighbor's routes from its table. If the

system receives a poll from its EGP neighbor, it responds with its own update packet.

Unlike the interior protocols discussed above, EGP does not attempt to choose the "best" route. EGP

updates contain distance-vector information, but EGP does not evaluate this information. The routing

metrics from different autonomous systems are not directly comparable. Each AS may use different

criteria for developing these values. Therefore, EGP leaves the choice of a "best" route to someone

else.

When EGP was designed, the network relied upon a group of trusted core gateways to process and

distribute the routes received from all of the autonomous systems. These core gateways were expected

to have the information necessary to choose the best external routes. EGP reachability information

was passed into the core gateways, where the information was combined and passed back out to the

autonomous systems.

A routing structure that depends on a centrally controlled group of gateways does not scale well and is

therefore inadequate for the rapidly growing Internet. As the number of autonomous systems and

networks connected to the Internet grew, it became difficult for the core gateways to keep up with the

expanding workload. This is one reason why the Internet moved to a more distributed architecture that

places a share of the burden of processing routes on each autonomous system. Another reason is that

no central authority controls the commercialized Internet. The Internet is composed of many equal

networks. In a distributed architecture, the autonomous systems require routing protocols, both

interior and exterior, that can make intelligent routing choices. Because of this, EGP is no longer

popular.



7.5.2 Border Gateway Protocol

Border Gateway Protocol (BGP) is the leading exterior routing protocol of the Internet. It is based on

the OSI InterDomain Routing Protocol (IDRP). BGP supports policy-based routing, which uses nontechnical reasons (for example, political, organizational, or security considerations) to make routing

decisions. Thus BGP enhances an autonomous system's ability to choose between routes and to

implement routing policies without relying on a central routing authority. This feature is important in

the absence of core gateways to perform these tasks.

Routing policies are not part of the BGP protocol. Policies are provided externally as configuration

information. As described in Chapter 2, the National Science Foundation provides Routing Arbiters

(RAs) at the Network Access Points (NAPs) where large Internet Service Providers (ISPs)

interconnect. The RAs can be queried for routing policy information. Most ISPs also develop private

policies based on the bilateral agreements they have with other ISPs. BGP can be used to implement

these policies by controlling the routes it announces to others and the routes it accepts from others. In

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